Volume 34, Issue 12, 01 December 1963
Index of content:
34(1963); http://dx.doi.org/10.1063/1.1729220View Description Hide Description
Plasma electron heating was accomplished by exciting plasma oscillations with a pulsed, 10‐kV, 1‐A electron beam drifting in a longitudinal magnetic field of 100 to 1000 Oe. The oscillatory energy gained by plasma electrons leads to an increase in their density and mean kinetic energy. The resulting plasma, which is called the ``beam‐plasma discharge,'' is an rf discharge in which a beam‐plasma interaction generates the rf fields.
At the beginning of a beam pulse, beam electrons generate a plasma by ionization of the background gas. Within a few microseconds excited plasma electrons become the dominant ionizationsource. This paper describes the transient rf oscillations that appear in the first few microseconds of the beam pulse. From a detailed study of this transient it was concluded that a nonconvective instability initiates the oscillations, which first appear at the electron cyclotron frequency, and that a convective instability at the plasma frequency generates the oscillations that sustain the discharge.
34(1963); http://dx.doi.org/10.1063/1.1729221View Description Hide Description
The dynamical equations which govern the simultaneous flows of heat and electricity in crystals express the electric and heat current densities as linear combinations of the gradients of the electrochemical potential and the temperature. When a magnetic field is applied, the coefficients in these equations become functions of the applied field. Magnetic fieldeffects are included by expanding the coefficients in powers of the magnetic field. For isotropic media, expansion of the electrical and thermal conductivities and the Seebeck coefficient to the first order in the magnetic field gives the Hall, Righi‐Leduc, and Nernst coefficients, respectively. The resulting dynamical equations have been solved assuming (a) ρ,κ,S are not functions of position or temperature, H≠0, current densityJ is either parallel or perpendicular to the temperature gradient ▿T and to the heat current densityQ; (b) ρ and κ not functions of position or temperature, H=0, J∥▿T.
Effect of Surface Polishing on the Linewidth and High‐Power Properties of Single‐Crystal Spheres of ZnY34(1963); http://dx.doi.org/10.1063/1.1729236View Description Hide Description
Single‐crystal (ZnY) (Ba2 Zn 2Fe12O22) was used in an investigation of the effects of surface finish on the linewidth and high‐power properties. Measurements indicate that by polishing rough‐ground spheres with successively finer grit abrasives, the linewidth can be reduced from 50 to 13.8 Oe. From the measured threshold field, the spin‐wave linewidth (ΔHK ) of 6.6 Oe has been calculated, in good agreement with that obtained by Green and Healy who used the parallel pump technique. Experiments indicate a threshold field an order‐of‐magnitude lower than for yttrium iron garnet with a comparable linewidth.
34(1963); http://dx.doi.org/10.1063/1.1729237View Description Hide Description
In the present paper, an analysis of laser action in semiconductors is made, and a condition for population inversion is established. The radiation output is found to be proportional to the deviation from the equilibrium value of the occupancy of the allowed states including the states in the conduction and valence band and also the donor and acceptor states. Possible laser levels are discussed.
Following the general analysis, a scheme is proposed for laser action in covalent semiconductors where the top of the valence band and the bottom of the conduction band do not occur at the same value of k. The scheme consists of simultaneously tunnelingelectrons into the (000) valley of the conduction band and injecting holes into the valence band. The outstanding feature of the proposed scheme is that it offers a definite possibility for separate amplitude and frequency modulation of the laser output. Various losses associated with the proposed scheme are also discussed.
34(1963); http://dx.doi.org/10.1063/1.1729238View Description Hide Description
The thermionic constants of UO2, UN, USi2, USi3, UB2, UB4, UC, UC–ZrC, and evaporated films of U on W have been determined using a pulsed strong‐field Schottky method within the temperature range of 1200° to 2400°K. The results are consistent with the hypothesis that a film of uranium on the surface activates all of these compounds. This surface film can be provided by a natural excess of uranium in the compound, from a chemical reduction by the base metal, or by evaporation from an external source. The resulting active state for all of the compounds can be described within a factor of 2 or 3 by the empirical Richardson constants Φ=3.0 eV, A = 10 A/cm2deg2.
34(1963); http://dx.doi.org/10.1063/1.1729239View Description Hide Description
An electron beam scanning technique has been devised to measure variations in the surfacework function. These measurements indicate a wide patch distribution in the range of 3.25 to 4.5 eV for inactive uraniumcarbide, which upon thermal activation becomes covered with a surface having a work function in the narrow band of 3.0 to 3.25 eV. The active surface is very sensitive to ambient gases and can be easily poisoned to a value of work function higher than was the case for the inactive state. These results support the hypothesis that a uranium film is responsible for the activation as was suggested by previous thermionic measurements. An analysis of the effect of a nonuniformity in work function on thermionic measurements, shows that experimental Richardson plots or effective work function plots can give highly erroneous results. The magnitude of this error can be determined and is expressed in terms of the spread of the patch distribution.
34(1963); http://dx.doi.org/10.1063/1.1729240View Description Hide Description
A hydrogen arc has been developed which produces a low‐density plasma column. The electrodes of the arc discharge are geometrically arranged such that the plasma drifts through a hole in the arc anode along a flux tube of the confining magnetic field.
A differentially‐pumped vacuum system was constructed which achieved a base pressure of 2.5×10−10 Torr in the inner liner region while at liquid‐nitrogen temperature. A density of about 1010 particles cm−3 can be achieved in the plasma column contained in the inner liner region, down field from the arc chamber, with a gas flow of 2 cm3 min−1 into the arc chamber. At this flow, the pressure in the inner liner rises to 3×10−9 Torr. The ion‐to‐neutral density ratio under these conditions is about 30 to 1 in the inner liner region.
34(1963); http://dx.doi.org/10.1063/1.1729241View Description Hide Description
A method is developed which permits a two‐fluid analysis of wave propagation in a magnetoplasma‐filled waveguide including the effects of Hall field, current inertia, space charge, unequal electron and ion temperatures, and diffusion.Maxwell's equations and the two‐fluid equations expressed in one‐fluid notation are reduced to a pair of coupled wave equations for the longitudinal components of the electromagnetic field. Approximate solutions are obtained for uncoupled TE and TM modes of propagation in a hydromagnetic waveguide. Included is a discussion of the effects of pressure, resistivity, displacement current, and current inertia on the dispersion relations.
34(1963); http://dx.doi.org/10.1063/1.1729242View Description Hide Description
A numerical method to solve Poisson's equation with an empirical set of boundary values gives potentials about complicated configurations such as occur in the lenses of electron microscopes. It involves solving an auxilliary induction problem and then solving the integral form of Poisson's equation numerically. The case for cylindrical symmetry has been coded on the IBM 7090 computer and a program developed that evaluates the electron optical properties of lenses possessing this symmetry.
34(1963); http://dx.doi.org/10.1063/1.1729243View Description Hide Description
The effect of a Maxwellian distribution of emission velocities on the longitudinal current density variation in axially symmetric solid beams is investigated to the paraxial approximation. The effects of both longitudinal and transverse initial emission velocities are included. The treatment permits variable magnetic fields and apertures which intercept some of the beam. Some particles may be turned back because all their energy is forced into the transverse motion; the reduction of the transmitted current density is evaluated. A simple illustrative example is given.
34(1963); http://dx.doi.org/10.1063/1.1729244View Description Hide Description
In the glow discharge in test diodes pulsed at high voltages and high duty cycles, spectroscopic analysis shows the presence of Ba, Sr, and Ca. It is concluded that the discharge is supported by vapors originating in particles of cathodematerial on the anode.
34(1963); http://dx.doi.org/10.1063/1.1729245View Description Hide Description
Atom ejection patterns from the Ge (100) and (110) surfaces have been studied for 100–800 eV Ne+, Ar+, Kr+, and Xe+ ion bombardment as a function of target temperature in the range 0°–350°C. For ion energies below a critical value dependent on the bombarding ion, atom ejection patterns are always observed; while for higher bombarding energies the patterns appear only when the target temperature exceeds a certain value. From these data an activation energy of 1.3 eV for diffusion of defects was obtained. The experimental results indicate that very little damage to the lattice results from ion bombardment when the ion energy stays below a critical value (Ar, Ne: ∼200 eV; Kr, Xe: ∼400 eV). At higher ion energy the damage becomes more severe and higher target temperatures are required for annealing the damage between subsequent ion collisions.
34(1963); http://dx.doi.org/10.1063/1.1729246View Description Hide Description
The secondary electron yield due to polonium‐210 alpha particles emerging from a thin goldsurface layer was determined to be 9.2 for an average alpha‐particle charge of 1.86. This result is consistent with yield observations with other high energy charged particles. The ratio of the yield due to fission fragments at normal incidence coming from UO2coatings to the yield due to alpha particles at normal incidence coming from gold was found to be about 20, substantiating the assumption that the secondary yield is proportional to the mass stopping power of the target material. A weak dependency of the yield on electric field was observed; an increase of approximately 8% resulted from a field of 3×105 V/m. A slightly more pronounced field dependency was observed for the yield from fission fragments.
34(1963); http://dx.doi.org/10.1063/1.1729247View Description Hide Description
Measurements are reported of the absorption, emission, and excitation spectra of Eu3+ in Y2O3, and also of fluorescence lifetime and quantum efficiency. An approximate level scheme is given. Conditions for laser action are calculated, and the results of a laser experiment are described. The importance of a strong ultraviolet excitation band probably corresponding to transitions from f‐electron to d‐electron levels is indicated.
34(1963); http://dx.doi.org/10.1063/1.1729248View Description Hide Description
34(1963); http://dx.doi.org/10.1063/1.1729249View Description Hide Description
The Poiseuille flow of a rarefied gas between two parallel plates is analyzed numerically for an inverse Knudsen number ranging from 0 to 10.5. The Bhatnagar, Gross, and Krook model is used and the transport integrodifferential equation is reduced to a purely integral one, which is solved numerically by the discrete ordinate method.
The plot of the volume flow rate vs pressure is shown to have the expected minimum; besides, it fits well with experimental results and previous approximate calculations.
In particular, the results given by Takao, properly corrected, are in good agreement with ours.
34(1963); http://dx.doi.org/10.1063/1.1729250View Description Hide Description
34(1963); http://dx.doi.org/10.1063/1.1729251View Description Hide Description
Improved low‐energy electron diffraction equipment which yields better resolution and a proper background is described. The equipment is sufficiently flexible for measurements of total elastic and inelastic backscattering, and this is illustrated with several examples.
Elastic and inelastic scattering factors are considered and illustrated by experimental data. Since they are complex, complicated, and only semiquantitatively understood, precise surface structure analysis is generally not possible at the current stage of development of this technique, though it now provides a very powerful method for characterizing surface structures. Various approximate methods for interpreting diffraction data are outlined. A preferred approach to a dynamic theory is also discussed.
Other physical properties such as Fresnel zone corrections, Lorentz factors, effects of contact potential,properties of diffraction by two‐dimensional gratings,properties of averages, specimen orientation effects, effects of steps, and thermal disorder are considered and illustrated. The analytical results for x‐ray and high‐energy electron diffraction are sueful but sometimes not directly applicable.
34(1963); http://dx.doi.org/10.1063/1.1729252View Description Hide Description
A mechanical fluctuation like Brownian motion takes place for colloidal particles of MgO and ZnO (particle size: 1000×1000×1000 and 5000×500×500 Å3) in a steady flow of electrons (10−5 A/cm2). This is observable as a broadening of the reflections in the electron diffraction patterns obtained with a satisfactorily short wavelength (0.026–0.028 Å).
The mean free path of MgO particles in recoil arising from collision with electrons was about 0.7 Å.
The observations permit not only the measurement of the mean weight of colloidal particles, but also demonstrate the quantum mechanical uncertainty principle.
34(1963); http://dx.doi.org/10.1063/1.1729253View Description Hide Description
Niobiumfilms 300 to 150 000 Å thick were formed by vacuum deposition using electron bombardment techniques. The films were superconducting at 4.2°K in magnetic fields as high as 30 kOe. Critical current density at 4.2°K is in the order of 106 A/cm2. The superconducting transition temperature, 9.3°K for a 2300‐Å thick film, is less for thinner films.Aging effects were noted for the high‐field current‐carrying capacity.